1. Field of the Invention
The present invention pertains to optical remote sensing systems, and, more particularly, to an evanescent wave-coupling used for Fresnel direction finding in such systems.
2. Description of the Related Art
LADAR and other laser guided airborne systems have historically used hemispherical radomes at their front end. The hemispherical shape was chosen to accommodate certain optical characteristics in the operation of the guidance systems. While increasing the performance of the guidance systems, they hampered the overall performance of the airborne system. One significant drawback to hemispherical radomes is that they create high levels of drag, which inhibit the speed of the airborne system. This is particularly undesirable in a military context because it renders the airborne system more vulnerable to enemy fire, thereby decreasing its survivability, and reduces lethality.
Efforts therefore were directed at developing new techniques that would accommodate the use of sleek, low drag radomes fostering speedier airborne systems. One technique developed as a part of this effort was non-coherent Fresnel direction finding (“NCFDF”). See, e.g., U.S. Pat. No. 6,851,645, entitled “Non-Coherent Fresnel Direction Finding Method and Apparatus”, issued Feb. 9, 2005, to Lockheed Martin Corporation as assignee of the inventors Brett A. Williams, et al. This technique ably accommodates the use of sleek radomes.
At the same time, some in the art have been pushing to decrease the size of such systems to obtain smaller, smarter, guided airborne systems. Desires for smaller guided projectiles, for instance, have produced pressure on sensors to fit within ever smaller dimensions. Opportunities for what may be termed “micro-missiles” such as darts or bullet-like projectiles in the neighborhood of 0.25″ diameters, or less, increase these strenuous demands still further.
While requirements for optical seekers conforming to sleek, high speed radomes vs. high drag, low speed hemispherical domes typically used by semi active laser (“SAL”) resulted in NCFDF with its independent, radially distributed windows and photodetectors, NCFDF begins to suffer for miniature missiles due to shrinking real-estate available for its window apertures and their associated lengths. The NCFDF apparatus in the patent mentioned above, for instance, uses a window/light-pipe design. The walls of the light-pipe are curved, reducing light rejection compared to straight walls. The aperture of the window collects light over its surface while the pipe simply guides light to its exit by multiple reflections. No focused, phase-front sensitive optics is required and wall reflection is most efficacious for total internal reflection without reflective coatings.
Light rejection and window field-of-view depend on a relation between aperture and light pipe length—that is, the aperture must be proportional to that length to avoid ever increasing reflection angles within the light-pipe until ultimately they are rejected back out to free space. If length limits are imposed, thus reducing allowable aperture dimensions, then additional apertures can be added which can satisfy the length relation for a smaller aperture/multi-window approach, increasing aperture area by the number of added windows. Yet micro-missiles have virtually no length allowance and even spacious airframes may have existing components in conflict with light-pipe placement.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.